We have demonstrated that circulating phosphate is a critical determinant of normal growth plate[unreadable] maturation. Hypophosphatemia impairs appptosis of hypertrophic chondrocytes, both in vivo and in[unreadable] vitro, leading to the development of rickets in growing animals. Interestingly, while hypophosphatemic[unreadable] Vitamin D Receptor (VDR) null mice, mice with diet-induced hypercalcemia/ hypophosphatemia and hyp[unreadable] mice all develop progressive expansion of late hypertrophic chondrocytes due to impaired apoptosis of[unreadable] these cells, Npt2a null mice have not been reported to develop rickets. Our preliminary data[unreadable] demonstrate that, while deletion of this renal Na-dependent phosphate transporter leads to the same[unreadable] degree of hypophosphatemia as is observed in the other models, the growth plate phenotype in these[unreadable] mice is transient. An expansion of the late hypertrophic chondrocyte layer, accompanied by impaired[unreadable] apoptosis of these cells is observed at 16 days of age, whereas by 35 days of age, there is resolution of[unreadable] this abnormality.[unreadable] The studies in this proposal will address the hypothesis that a specific transport mechanism for[unreadable] phosphate is present in hypertrophic chondrocytes and mediates apoptosis of these cells. They will[unreadable] examine whether there is an intrinsic difference in phosphate transport in cultures of hypertrophic[unreadable] chondrocytes from hyp mice, Npt2a null mice and wildtype mice. They will also determine whether[unreadable] intrinsic cellular factors modify the susceptibility of chondrocytes, isolated from these animals, to[unreadable] phosphate-mediated apoptosis.[unreadable] Investigations will be performed to address the hypothesis that a difference in circulating hormone[unreadable] levels contributes to the difference in the growth plate phenotype of the Npt2a and hyp mice. Primary[unreadable] chondrocytes, isolated from these mice will be treated with PTH, 1,25-dihydroxyvitamin D and FGF23 to[unreadable] address the hypothesis that these agents modulate phosphate transport and/or susceptibility to[unreadable] phosphate-mediated apoptosis. To address the hypothesis that enhanced 1,25-dihydroxyvitamin D[unreadable] action is responsible for normalization of the growth plate of Npt2a null mice, 1,25-dihydroxyvitamin D[unreadable] action will be blocked by mating them with VDR null mice. Hyp mice will be treated with 1,25-[unreadable] dihydroxyvitamin D to address the hypothesis that impaired 1,25-dihydroxyvitamin D action and /or[unreadable] increases in PTH contribute to the development of rickets in this disorder.[unreadable] These investigations are expected to reconcile the difference in the growth plate phenotype[unreadable] observed in hypophosphatemic mouse models and to identify a role for circulating hormones in[unreadable] modulating the susceptibility of hypertrophic chondrocytes to phosphate-mediated apoptosis.